1. Field of the Invention
The present invention relates to a semiconductor laser, segmented into lasing elements, whose emitted beam elements may be combined into a single beam of light and, more particularly, to an array of semiconductor lasers containing an additional optical element which modifies the percentage of light which propagates into a diffractive order. Emphasis is placed on vertical-cavity surface-emitting lasers.
2. Description of the Prior Art
Semiconductor laser diodes, in general, comprise a body of a semiconductor material having adjacent regions of opposite conductivity type forming a p-n junction therebetween. The body is adapted to generate and emit radiation when an appropriate potential is applied across the p-n junction. Vertical-cavity, surface-emitting lasers (VCSELs) emit radiation in a direction perpendicular to the plane of the p-n junction or substrate rather than parallel to the plane of the p-n junction as in the case of conventional edge-emitting diode lasers. In contrast to the elliptical and astigmatic beam quality of conventional edge emitting lasers, VCSELs advantageously emit a circularly symmetric Gaussian beam and thus do not require anamorphic correction. VCSELs moreover, may be readily made into two-dimensional laser arrays as well as may be fabricated in extremely small sizes. Accordingly, two-dimensional VCSEL arrays have various applications in the fields of optical memory, laser printing and scanning, optical communications, optoelectronic integrated circuits, optical computing, optical interconnection, etc. Some applications require high power beams to be focused into a single spot. The power required is much larger than the power readily obtainable from a single-transverse-mode VCSEL. Thus, attempts have been made to operate a large array of VCSELs, each emitting a beam element, and combine the beam elements into a single beam of light which may be focused into a single spot.
There are three basic requirements for combining an array of beam elements of light from an array of lasers into a single beam of light which may be focused into a single spot. The first requirement is that the phases of the light waves from each beam element must be strongly phase coupled. The second requirement is that the phase difference between the light waves from beam elements must be close to zero. The third requirement is that the diffraction, caused by the structure of the array of lasers, must be controlled. While the prior art describes attempts to accomplish the first two stated requirements, it is absent of techniques to accomplish the third. Because phase coupling between laser elements is stronger in two-dimensional array configurations than it is for one-dimensional arrays, the discussion will concentrate on two-dimensional arrays. It should be appreciated that this discussion is also generally applicable to one-dimensional as well as multi-dimensional arrays. Since VCSEL technology represents the most practical means for fabricating two-dimensional arrays, the discussion furthermore concentrates on VCSELs. However the results are also valid for any type of semiconductor laser.
An effective technique for fabricating a phase-coupled array of VCSELs is described in an article by M. Orenstein et al., entitled "Large two-dimensional arrays of phase-locked vertical cavity surface emitting lasers," published in Applied Physics Letters, Vol. 60 (13) (Mar. 30, 1992), pgs. 1535-1537. In this technique, VCSEL elements are separated by a square grid of patterned metal over a wide aperture, with each VCSEL element on the order of 10 micrometers across. Although the laser elements are phase coupled, adjacent laser elements have a 180 degree phase difference between them. This "out-of-phase coupling" results in the propagation of four beams of equal intensity, rather than the desired single beam. The technique thus satisfies only the first of the above-stated requirements. It is well known that most phase-coupling techniques, such as this, result in out-of-phase coupling and thus produce four-beam propagation for two-dimensional arrays, and two-beam propagation for one-dimensional arrays. An improvement upon this technique is described in an article by M. E. Warren et al., entitled "On-axis far-field emission from two-dimensional phase-locked vertical cavity surface-emitting laser arrays with an integrated phase corrector," published in Applied Physics Letters, Vol. 61 (Sep. 28, 1992), pgs. 1484-1486. In this technique the laser elements are allowed to couple out of phase. A phase plate on top of the laser array then produces a 180 degree phase shift of half of the elements, bringing all laser elements in phase with one another. This technique, therefore, additionally satisfies the second requirement for combining beam elements into a single beam. The experimental demonstration by Warren et al. was idealized somewhat in that the laser was optically pumped, rather than electrically pumped as is preferred. Despite the idealized configuration, only about 50% of the light propagated into a single diffractive order. The remainder of the light propagated into other diffractive orders. The diffraction of a significant amount of the light into multiple diffractive orders will render the laser array either inefficient or impractical for nearly all desired applications.
The diffraction pattern published by Warren et al. strongly resembles the pattern which would be expected if one illuminated a square-pattern diffraction grating with a coherent beam of light. When the amplitude of the light in a phase-coupled array is considered as well as the phase, diffraction of the phase-coupled light into higher diffractive orders is expected. The light amplitude must be zero at the grid lines which separate the light elements, thus an effective amplitude grating is formed by the characteristics of the array of lasers.
There are additional prior art methods for combining beam elements into a single beam. One such technique utilizes a Talbot cavity in which the laser cavity is extended in order to phase couple the lasing elements as applied to VCSELs and described in an article by Ho et al., "Effective reflectivity from self-imaging in a Talbot cavity and its effect on the threshold of a finite 2-D surface emitting laser array," published in Applied Optics, Vol. 29 (34) (Dec. 1, 1990), pgs. 5080-5085. Another method for phase coupling lasing elements is to use "antiguides," alternatively termed "leaky mode coupling." Antiguiding is discussed for VCSELs in an article by Hadley, "Modes of a two dimensional phase-locked array of vertical-cavity surface-emitting lasers," published in Optics Letters, Vol. 15 (21) (Nov. 1, 1990), pgs. 1215-1217. The simultaneous use of antiguides with a Talbot cavity for edge-emitting semiconductor lasers is described in an article by Botez et al., "Watt-range, coherent, uniphase powers from phase-locked arrays of antiguided diode lasers," published in Applied Physics Letters, Vol. 58 (19) (May 13, 1991), pgs. 2070-2072.
The prior art experiments and simulations show light diffracted into multiple diffractive orders. The prior art does not address the problem of diffraction of light into multiple orders however, nor do they suggest a method or apparatus for correcting this diffraction problem.